When it comes to disease causes, what experts tell the press and what the press tells the public often have little in common. Consider how the media reported a recent finding that heredity plays almost no role in developing Parkinson’s disease after age 50.

The study, published in January by the Journal of the American Medical Association (JAMA), made national news. Almost every one of the newspaper stories emphasized the potential role of "pesticides and herbicides" in triggering the disease, repeating it from piece to piece like a mantra.

It’s unsettling that so many reporters, often self-identified as science or medical writers, don’t know that "herbicide" — a weed killer — is a subcategory of pesticide. More important, it’s a classic illustration of pack journalism.

In fact, the JAMA study made no mention of pesticides and referred to just one chemical — a synthetic heroin substitute called MPTP known to induce the trembling associated with Parkinson’s. The study design didn’t even allow researchers to look at any cause other than genetic.

So what’s with all this "herbicide and pesticide" talk?

At a news conference, the JAMA authors stated the obvious — if the cause of the disease isn’t genetic, then it must be environmental. "Environmental" can mean pesticides. But it can also mean anything from ingested foods and liquids to germs and weather patterns. In Muhammad Ali’s case, it may have been exposure to leather — a fabric found often in boxing gloves.

As for chemicals being the "main cause" of Parkinson’s, Dr. J. William Langston of the Parkinson’s Institute in Sunnyvale, Calif., a senior author of the JAMA study, says he and his colleagues were silent on the question.

Still, Langston said, "There are quite a number of epidemiological studies showing risk related to pesticide and herbicide exposure and Parkinson’s, but no smoking gun."

The usual suspects are the herbicides paraquat, which protects corn, grapes, citrus fruits and other crops, and diquat dibromide, mostly used for controlling aquatic weeds and protecting potato vines. Both are sold in the U.S. by Zeneca Inc.

It’s often claimed those herbicides have a similar composition to MPTP. But, "These compounds are chemically different and they behave very differently in the body," said Ted Lock, a chemist with the Zeneca Central Toxicology Laboratory near Manchester, England.

For one, MPTP is fat soluble. That means it can readily cross the body’s shield between the blood and the brain. But the "quats" are water soluble and cannot. Efforts to pump enough into a lab rodent to penetrate the blood-brain barrier end up killing the poor thing outright.

And MPTP is a "non-charged" compound. This has nothing to do with paying cash for it. Rather, "Once inside the brain (the chemical) undergoes a transformation," explained Lock, and "mimics a naturally-occurring compound called dopamine." This kills cells, slowly degenerating the nervous system.

Considering his employer, Lock may be biased. But published medical literature — and common sense — backs him up.

"That’s bull," said Dr. William Landau, former chief neurologist at the George Washington School of Medicine in St. Louis, about efforts to blame Parkinson’s on pesticides. "In 1817, William Parkinson described the disease long before current pesticides were in use." Paraquat and diquat weren’t introduced until the early 1960s.

Further, "Parkinson’s is found in every culture of the world," notes Landau. "We don’t know what causes the disease. But attributing it to something because people have a compulsive drive to postulate a cause to explain the unknown — despite not an iota of evidence — is remarkably stupid."

There is no evidence of any increase in Parkinson’s in the era of modern pesticides, which began after World War II. And there are no reliable national data, since the disease isn’t required to be reported.

But the Mayo Clinic in Rochester, Minn., has maintained the world’s oldest database on Parkinson’s disease, monitoring area residents since the 1930s. It shows no increase in Parkinson’s once the aging of the population is factored in. Though, naturally, it can never be ruled out that any given chemical might cause unusual effects on certain individuals.

Bottom line: If you’re really worried about losing brain cells, don’t sweat the pesticides. Just stop reading the paper.

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Read Michael Fumento’s additional work on the media, on disease, and on pesticides.